America's most famous sex therapist has warned that social media is propelling millennials into a future of loneliness in which they will have difficulty forming lasting relationships.

Dr Ruth Westheimer, universally known as 'Dr Ruth', said she was deeply concerned that young people would not find marriages that lasted.

She had one exception: Harry and Meghan. She said the royal newly-weds were clearly so in love they were one of the few couples that did not need her advice.

Dr Ruth, who turned 90 last week, said: "Of course I watched the royal wedding because I am very interested in relationships."

But she added: "It's much more difficult these days to keep a relationship going - and it's because of the internet. It gives people unreasonable expectations, like that a Prince Charming will come riding in on a horse."

The 4ft 7in sex therapist said that addiction to social media meant "loneliness" was going to become a bigger problem than "sexual literacy", which she used to guide the nation on.

"I'm very concerned about loneliness," she said. "People are losing the art of conversation. They're constantly on their iPhones. It's a big problem for the relationships."

To that end Dr Ruth is returning to TV with a programme aimed at millennials. She will have a young, male co-host and has promised to let him get a word in.

"What the young people need now is a voice like mine, a little bit old-fashioned and square," she said.

"We need to tell them to put the phone aside, and put their energy into forming relationships. You can't ignore the phone, but you have to know not to be addicted."